Sonar is a well known apparatus having both civilian and military applications. Sonar (originally an acronym for SOund Navigation And Ranging) is a technique that uses sound propagation, usually underwater, to navigate, communicate with or detect other vessels. Sonar uses sensors placed in arrays to receive sound. The arrays can be deployed in many ways. Some sonar arrays are towed behind a ship or submarine. Towing an array of sensors or hydrophones presents many problems. Amongst the problems are keeping the tow lines straight during vessel maneuvers. Another way to deploy an array is by mounting sensors to the hull of a ship, such as a submarine. Hull mounted sonar arrays are generally built up from separate components at several hull mount sites on a hull. Typically, there are a number of hull mount sites that are aligned along the starboard side of the hull and an equal number of hull mount sites aligned along the port side of the hull. Each hull mount site includes a baffle, a signal conditioning plate (also referred to as an SCP), a vibration isolation module (also referred to as a VIM), an array of sensors, and an outer decoupler (also referred to as an ODC). As mentioned, each of these separate components is placed on the hull one after the other. The building process is time consuming as it takes time to build up each site. In addition, many of the separate components are bulky and heavy.
The signal conditioning plate is attached to the baffle. The signal conditioning plate bounces incoming signals back towards the wet-side to the mounted sensors to produce a reflection gain at the sensors in the array. The signal conditioning plate is made of materials so that it can be tuned to produce gain in the frequency of interest.
In order to provide signal enhancement of the incident signals, baffles have been developed to improve the signal-to-noise ratio on hull mounted sonar arrays. Baffles tend to prevent hull noise. Also, in order to achieve this desired result, outside decouplers in the past have been designed to perform two functions, namely: (1) to provide, in conjunction with a signal conditioning plate, the proper impedance backing for one or more hydrophones included in the array; and (2) to isolate or decouple flow noise from the incident signals which tends to undesirably degrade the overall performance of the system.
With regard to the first function, an ideal signal conditioning device is one which when placed directly behind the hydrophones operates to enhance the signal response at all frequencies without introducing phase shifts. In known prior art apparatus, thick steel plates having pressure release, i.e. low impedance, backings have been used to approach this end. However, as the need for improved performance requires the use of lower and lower operating frequencies, the thicknesses and weight requirements for the steel plate structures have become prohibitive from a practical standpoint.
The vibration isolation module is attached to the signal conditioning plate. The vibration isolation module provides attachment points for the array of sonar sensors that decouple the array from the normal hull vibrations. The vibration isolation module main purpose is to substantially prevent or lessen unwanted noise from vibrations of the hull from reaching the sensors.
Hull-mounted acoustic array panels typically require a sensor module architecture customized for the host platform in question. Existing hull-mounted acoustic array panels use either pressure sensors or accelerometers, along with a baffle/SCP tuned to that particular element. The customization required adds to the cost of the acoustic array panels.
Hull-mounted acoustic array panels typically mount a sensor module onto the Signal Conditioner Plate (SCP). The electronics associated with the sensor (amplifiers etc.) typically reside in an off-board “bottle” which lies to the side, or on top of, these modules. In order to form a continuous array of sensors, this architecture dictates either two rows of column-based stave modules, or two columns of row-based ones. This basically means that each platform requires a separate set of custom stave modules to span the desired acoustic aperture. In addition, the staves of sensors are wired so that if one sensor fails, the remaining sensors in the stave also fail.
In many instances, the individual sensors are made from solid ceramic plates or solid ceramic blocks and so are also heavy. Heavy sensors result in a heavy array of sensors. The heavy arrays add to the weight to the assembly needed for a hull mounted array. The staves are also wired together and with signals being carried out to rails on the side of the sensor array. The stave architecture is also somewhat inflexible. Each set of staves is custom designed for each platform on which the sensors are placed. When attaching staves of sensors to other platforms, a new custom design is made specific to the platform.
As mentioned previously, the current panels are heavy. The baffle that forms part of the acoustic array panel is one of the heavier portions of the panel. The individual sensors include a solid ceramic plate or solid ceramic blocks and add to the weight of the panel.
In ship building, it is a constant goal to make the vessel lighter. Another goal is to make components more reliable. Still a further goal includes making the components easier to install.
In addition, all sonar, radar, and optics systems receive waveforms from turbulent diffracting and refracting environments. Not all portions of a wavefront pass through the medium of propagation with identical results because all real-world mediums are not homogeneous. The small differences in ray path characteristics have the tendency to distort the wavefront (i.e. make it non-spherical). In current systems, distortions in the wavefront are corrected after the sound wave has passed the sensors, such as an acoustic sensor in a SONAR system. In such a device, electronics are used to correct for distortion of the wavefront after it has passed. In other words, the electronics are always playing catch-up because the signal conditioning performed to correct for distortion is done after a wave has already passed. In such a system an assumption must be made that may not necessarily be true. The assumption is that the random variations in distortion are not independent events and, therefore, vary slowly compared to the time it takes to adjust the sensor. This is probably not a terrible assumption, but an imperfect assumption, nonetheless.